To achieve high manufacturing yields, the semiconductor industry depends on careful inspections of photomasks, bare silicon wafers, and processed silicon wafers. The inspection requirements are becoming more stringent as the industry inscribes smaller features on each new generation of integrated circuits. The resolution of optical inspection systems is becoming inadequate to find the small defects which can spoil the performance of an integrated circuit. To find smaller defects, the industry will increasingly rely on electron-beam inspection systems and review stations, which offer at least 10 times better resolution than optical systems. These inspection systems are variations on the traditional scanning electron microscope. Such inspection systems and review stations may, of course, find use in evaluation of other samples, such as biological samples, metallurgical samples, and the like.
Scanning Electron Microscopes
In a scanning electron microscope (SEM), a beam of electrons perhaps 3.5 nanometers in diameter scans across the surface of a substrate (for example, a photomask, a bare silicon wafer, a processed silicon wafer, or another sample). The electrons in the beam, known as “primary electrons,” penetrate the substrate and dislodge electrons within the material. Some of these dislodged electrons, known as “backscattered electrons” or “secondary electrons,” escape from the surface of the substrate.
A detector captures the emitted electrons. The SEM's electronic imaging system can transform the detector's output into a black-and-white image of the surface. Darker areas of the image correspond to areas on the substrate which emitted fewer electrons, and lighter areas of the image correspond to areas of the substrate which emitted more electrons.
SEM-Based Inspection Systems
When an SEM-based inspection system acquires an image of a region on a wafer or a photomask, it has no immediate way to know whether the pattern in that region is correct. To find defects, the system typically captures not only an image of the region to be inspected (the “test image”) but also a reference image, then compare the two with high-speed electronics. In “die-to-die” mode, the reference image is a corresponding area on a nominally identical die on the same wafer or photomask. In “array mode,” the reference image is a nominally identical pattern in a different location on the same die, for example, on DRAM chips or on photomasks for exposing DRAM chips.
The system electronically aligns the test image with the reference image, than compares them to look for significant differences. Most systems execute the comparison by assigning a numerical gray scale value (from, say, 0 to 256) to each pixel, then subtracting the gray scale values of the corresponding pixels in the two images. They can calculate and display a “difference map” or “defect map” in which the gray scale value of each pixel is the difference between its value in the test image and its value in the reference image. Defects can appear as bright areas in the difference map.
The Defect Threshold
One might expect that the difference in gray scale value would be zero for most of the pixel pairs if the sample contains few real defects. However, slight differences in gray scale values between corresponding pixels in the test and reference images occur frequently due to various sources of system noise and to processing-induced pattern variations which might be too subtle to impair circuit performance.
The designer and/or the operator of the inspection system have to define a “defect threshold,” i.e., a difference in gray scale values. If corresponding pixels in the test and reference images differ in gray scale value by an amount less than the threshold, the system will ignore them. If they differ by an amount equal to or greater than the threshold, the system will report a defect. In that case, the system may also scan a third nominally identical region, an “arbitrator” to determine whether the defect lies in the test image or in the reference image. The system concludes that the image which matches the arbitrator is correct and the one which differs contains the defect.
The selection of a defect threshold involves a complex and serious engineering tradeoff between sensitivity and false or nuisance defects. If the defect threshold is set very low, the system will be more sensitive; it will almost certainly find all the significant defects which could impair circuit performance. However, it may find thousands of “false defects,” areas in which the patterns are actually identical, but the gray scale values differ because of system noise. It may also find thousands of “nuisance defects,” areas in which the patterns differ but the differences are small enough to ignore. The operator may have to review thousands of defect reports to cull the real defects from the false and nuisance defects.
If, on the other hand, the defect threshold is set very high, the system will report few false defects and few nuisance defects. However, it will be less sensitive, and it might miss a critical defect which can ruin the circuit.
The Effect of Detector Position
In early generations of SEM's, a single detector, often referred to as a “total-yield detector,” captured virtually all the backscattered and secondary electrons, no matter what their initial trajectory, with help from electrical or magnetic fields. Since the rate of secondary electron emission depends sensitively on the materials and on the topography, these instruments could produce high-resolution images of, for example, metal lines on an oxide or quartz surface. A printed image would easily reveal defects such as an extra metal line, a missing metal line, a broken metal line, etc.
However, instruments with a single total-yield detector are less successful at revealing variations in surface topography; i.e., at finding areas which rise above or dip below the plane of the surface. For example, during chemical-mechanical polishing of a silicon wafer, a dust particle or an impurity in the slurry can create a “microscratch,” an indentation which might be 0.1 micron wide by 0.1 micron deep by 1 micron long. A microscratch can be a critical defect in an integrated circuit because metal can fill it, creating an electrical short between two lines that are supposed to be isolated. A total-yield detector can't detect the microscratch on the basis of materials contrast because the microscratch is just a tiny gouge within one material.
To detect microscratches and other small variations in surface topography, we can get them to cast “shadows” in an SEM image by using a detector which selectively collects secondary electrons moving toward one side or the other. This “shadowing” will occur in any system which does not produce a point-to-point image of the sample on the detector, i.e. electrons emitted from the same point but at different angles from the sample will arrive at different positions on the detector. For example, the instrument might contain a detector positioned to the side of the substrate just above the plane of the substrate surface, and strategically placed electric or magnetic fields might direct only the electrons moving toward that side into the detector. The resultant shadows make it easier to see microscratches and other variations in surface topography.
A detector in a scanning electron-beam will find certain defect types with greater or lesser sensitivity, depending on its elevational angle. Mounted overhead, it will be more sensitive to differences in materials; mounted to the side, it will be more sensitive to microscratches and other variations in surface topography. To optimize sensitivity for many defect types, some SEM inspection systems rely on two detectors: one mounted overhead, one mounted to the side.
Prior Art SEM-Based Inspection Systems with Two Detectors
The presence of two or more detectors raises the question of how to apply the information which a plurality of detectors provides. One prior art method uses the signal from the second detector to provide a “cross-check” for the signal from the first detector.
In this prior art method, each of two detectors in the inspection system looks at secondary electrons from two nominally identical regions. For clarity, we refer to the two detectors as Detector A (mounted on top) and Detector B (mounted on the side) and assume it's a die-to-die inspection of a processed silicon wafer.
Detector A takes an image of a specific region on a die (the test image) and an image of a nominally identical region on an adjacent die (the reference image). For each pair of corresponding pixels, the system subtracts the gray scale values and compares the difference with the threshold. If the difference is below the threshold, the system ignores them. If the difference equals or exceeds the threshold, the system reports a defect. The one-dimensional plot in FIG. 1a illustrates this part of the method.
Detector B repeats the process: it takes an image of the same two regions imaged by Detector A. For each pair of corresponding pixels, the system reports a defect only if the difference in gray scale values exceeds the threshold value. The one-dimensional plot in FIG. 1b illustrates this part of the method.
One significant weakness of the method disclosed by the prior art is that it attempts to distinguish real defects from false or nuisance defects on the basis of data from a single detector. It identifies defects first on the basis of two sets of image data taken by Detector A, then on the basis of two sets of image data taken by Detector B. It describes a data processing technique that essentially uses the two one-dimensional plots shown in FIG. 1. This prior art method doesn't acknowledge or recognize any advantage to be gained from combining more than two data sets in innovative ways before defining a defect. As a result, the prior art doesn't ameliorate the difficult tradeoff between sensitivity and false defects. Furthermore, it lends the operator very little flexibility in terms of his ability to look selectively for certain defect types.
In another prior art method, signals from a backscattered electron detector and a secondary electron detector are compared to produce information concerning the location, size and shape of features on a substrate. This prior art method makes use the unique characteristics of the secondary and backscattered electron waveforms to provide additional information concerning the surface under inspection.
In yet another prior art method, signals from two different detectors are combined to produce a composite image of a high aspect ratio structure. The detectors may be separately optimized for imaging the top and bottom, respectively, of the high aspect ratio structures. The resulting image may have an extended focus.
Plural detectors have also been used in the prior art to produce composite images in which differences in electron trajectory or position are represented in a color display. The positions of such detectors have been varied about the specimen, and differences between the signals from these detectors have been analyzed in such systems.
None of the aforementioned prior art methods takes full advantage of the capabilities of a multi-detector SEM to inspect substrates.